1. Field of the Invention
The invention relates to substrate measurement that is performed on a front side or a back side of a substrate.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
During production of integrated circuits, a substrate is typically fed into a lithographic apparatus several times in order to be able to produce a circuit which consists of several layers on top of each other. As many as 30 layers can be used. The lithographic apparatus used to generate the circuit pattern in the first layer is typically not the same as the lithographic apparatus used to generate the circuit pattern in the final layer. This is because the features of the circuit pattern in the final layer are typically much larger than the features in the first layer, so that a less accurate and therefore less expensive lithographic apparatus can be used to apply the desired circuit pattern to the final layer.
Conventionally substrates are provided with alignment marks whose positions relative to the target portions are known. During alignment, an alignment sensor measures the positions of the alignment marks. In this way, the positions of the target portions may be determined. The alignment sensor views a small area on the substrate at a given time, the small area being considered the footprint of the alignment sensor. Often, when alignment is begun, the alignment mark does not coincide with the area viewed by the alignment sensor. To solve this problem, the substrate is scanned underneath the alignment sensor, over a distance sufficiently large to make certain that the alignment mark passes through the area viewed by the alignment sensor. The position of the alignment mark is measured as it passes through the area viewed by the alignment sensor.
A possible disadvantage of this alignment method is that scanning of the substrate is time consuming, and thus effects the number of substrates that can be processed by the lithographic apparatus per hour.
Instead of using alignment marks, U.S. Pat. No. 3,898,617 describes an alignment system that measures positions of circuit features so that alignment is performed using circuit features. The alignment sensor views a small area of a target portion on the substrate. The sensor records an image of the circuit features located in the area of the target portion. The image is compared to a library which includes images of circuit features and their associated locations. When a match is found between the measured image and a library image, the associated location retrieved from the library gives the location of the substrate.
A possible disadvantage of this method is that a time consuming scan of the substrate underneath the alignment sensor may be needed until a recorded image is found which matches an image in the library.
Once alignment is achieved, several lithographic apparatuses may be involved to simultaneously apply the desired circuit pattern onto the first layer or any other layer of the substrate. Even though the calibrations of the machines are performed as accurately as possible, each apparatus may introduce its own errors. These errors may adversely affect the image applied to the substrate or the position of the image on the substrate. In the event a lithographic apparatus is calibrated between two sets of substrates (commonly referred to as lots), the error may also differ for the two sets.
When two lithographic apparatuses are used simultaneously, this implies that there may be several patterning structures available containing the pattern to be applied to a given layer. The several patterning structures may also differ due to production tolerances. These differences may lead to differences in the images applied to the substrate or to differences in positions on the substrates where the images are applied.
Commonly the substrates are marked with a code that is scratched into the substrate. The substrates can be identified using these codes. The relation between the identities of the substrates and the lithographic apparatuses or the patterning structures used to project images onto the substrate is stored. The combination of the stored relations and the identity of the substrates can be used to correct for the differences, based upon knowledge of the previously used lithographic apparatus or patterning structure.
However, a special sensor is required in the lithographic apparatus to read the code in order to identify the substrate or determine the process steps applied to the substrate. This has effects on the costs of the lithographic apparatus and on the throughput since time is needed to read the code. Given the relatively high cost of a lithographic apparatus compared to the other machines used during the total production of integrated circuits, there are a limited number of lithographic apparatuses available to the system, so the throughput of the lithographic apparatus is typically the bottleneck in the production process.
In addition, placing codes on substrates has been limited because the space used to print the codes takes away from the valuable space that may be used to manufacture integrated circuits. In order to increase efficiency and reduce cost, it is desirable to manufacture additional integrated circuits on the substrate without increasing the size of the substrate. Producing integrated circuit may be made less expensive per integrated circuit or faster per integrated circuit when more integrated circuits are placed on one substrate. Therefore, the production costs can be decreased and the throughput can be increased by freeing space on the substrate for extra integrated circuits and refraining from reserving space of codes.
Known methods for increasing substrate target areas have included placing alignment marks on a back side, or second side, of the substrate, which is opposite to a front side, or first side, of the substrate. Typically, the front side includes the integrated circuit. A lithographic apparatus that includes an optical system which is capable of directing alignment radiation to the back side of the substrate is disclosed in U.S. Pat. No. 6,768,539, herein incorporated by reference in its entirety. The image of the alignment mark may be provided at a plane of the first side of the substrate. This enables a common alignment system to be used for alignment of marks on both sides of the substrate. The alignment system may be capable of performing alignment using features on the front and back of a substrate.